Continued adoption of cellular systems for data transfer as well as voice communications along with introduction of new mobile communications devices such as Tablet devices make cellular coverage in urban environments a priority. In particular, improving cellular coverage in public venues where large a number of cellular users are present is important to provide a seamless user experience in the mobile communication arena. Distributed antenna systems (DAS) are being installed in sports arenas, convention centers and other public areas and are used to provide stronger RF signals to improve the communication link for cellular and data services. These DAS systems are crucial to maintaining capacity of cellular systems as users increase the amount of data that is uploaded and downloaded.
Initial DAS antenna systems were only required to operate over a few frequency bands, making the antenna design process easier. As the communications industry has moved from 2G to 3G cellular systems, frequency band count for DAS antennas has increased. With the advent of 4G communication systems such as Long Term Evolution (LTE), additional frequency bands are required from a DAS antenna system which increases the difficulty in terms of antenna design. LTE also brings a two antenna requirement needed to implement a Multiple Input Multiple Output (MIMO) antenna system.
As the density of mobile communication users increases in public spaces such as sports arenas and convention centers, and as more users access high data rate features such as file sharing and video downloads the signal to noise characteristics and RF signal levels of the cellular signals indoors become more important parameters. To maintain low noise floors in communication systems a parameter that is important to address in the antenna design is Passive Intermodulation (PIM). PIM products are generated when two RF signals at different frequencies are injected into an antenna port; the antenna, though being a passive device, can generate spurious responses due to “natural diode” junctions in the antenna. These natural diode junctions can be formed at the junction of two metal surfaces where the metals are dissimilar. Corrosion and oxidation at these junctions can also cause spurious frequency components due to mixing of the two RF signals. Proper antenna design and material selection is important to meet stringent, low PIM requirements. As PIM components increase, these spurious frequency components add to the noise level, which in turn results in reduced signal to noise ratio of the communication system. This will result in reduced data rates for users.
Low PIM requirements can be difficult to obtain in high gain antenna applications due to the large number of antenna elements typically used to form an array. Arraying multiple antenna elements together is a common technique used to generate higher gain antennas. The multiple connection points required when arraying multiple antennas together provide more opportunities for PIM to be produced; these connection points can be antenna element to ground plane connections, connector to ground plane interfaces, and coaxial transmission line interfaces.
Initially, low gain antennas were used implement DAS systems in public venues. To maximize capacity as the number of mobile users increased at public venues, higher gain antennas were used to replace the low gain, near omni-directional antennas. These higher gain antennas are typically a linear array of elements which provide a narrow or reduced beamwidth in one plane while maintaining a broad or wide beamwidth in the other principal plane passing through the main lobe of the radiation pattern. As the density of mobile communication users increases further there is a need to move from antennas that have a narrow beamwidth in one plane to narrow beamwidth in both principal planes of the radiation pattern. This move to full 2D arrays will bring more complexity to the arraying process as well as complexity in regards to maintaining PIM performance.